Use a less memory intensive method to check if a node has already been bound

Author: jakebailey

src/compiler/binder.ts

@@ -227,7 +227,7 @@ namespace ts {
         const reportedUnreachableFlow: FlowNode = { flags: FlowFlags.Unreachable };
         const bindBinaryExpressionFlow = createBindBinaryExpressionFlow();
 
-        const alreadyBound = new Set<Node>();
+        let alreadyBound: WeakSet<Node>;
 
         /**
          * Inside the binder, we may create a diagnostic for an as-yet unbound node (with potentially no parent pointers, implying no accessible source file)
@@ -245,6 +245,7 @@ namespace ts {
             inStrictMode = bindInStrictMode(file, opts);
             classifiableNames = new Set();
             symbolCount = 0;
+            alreadyBound = new WeakSet();
 
             Symbol = objectAllocator.getSymbolConstructor();
 
@@ -282,7 +283,6 @@ namespace ts {
             hasExplicitReturn = false;
             inAssignmentPattern = false;
             emitFlags = NodeFlags.None;
-            alreadyBound.clear();
         }
 
         return bindSourceFile;